OpenMP github action runners are oversubscribed

[hughcars]

The github action runners for openmp are oversubscribed, due to the test matrix tests assuming with-openmp is a boolean, whereas it is a string. The string makes the job descriptions easier to parse in CI, but are causing this error:

Run # Configure environment
....
  if [[ "$NUM_PROC_TEST" -gt "$NUM_PROC_TEST_MAX" ]]; then
    NUM_PROC_TEST=$NUM_PROC_TEST_MAX
  fi
  if [[ "serial" == 'true' ]]; then # Problem!!
    NUM_PROC_TEST=$(( NUM_PROC_TEST / 2 ))
    export OMP_NUM_THREADS=2
  else
    export OMP_NUM_THREADS=1
  fi